1. Field of the Invention
The invention relates to methods, systems and structures for removing ice from surfaces.
2. Statement of the Problem
Ice adhesion to certain surfaces causes many problems. For example, icing on power lines adds weight to the power lines causing them to break. In addition to the costs of repair, the resulting power outages cause billions of dollars in direct and indirect economic damage. The large surface areas of power lines exposed to icing conditions and the remoteness of many lines require de-icing systems that are both reliable and have low costs per unit distance.
Excessive ice accumulation on aircraft wings endangers the plane and its passengers. Ice on ship hulls creates navigational difficulties, the expenditure of additional power to navigate through water and ice, and certain unsafe conditions. The need to scrape ice that forms on automobile windshields is regarded by most adults as a bothersome and recurring chore; and any residual ice risks driver visibility and safety.
Icing and ice adhesion also causes problems with helicopter blades, and with public roads. Billions of dollars are spent on ice and snow removal and control. Ice also adheres to metals, plastics, glasses and ceramics, creating other day-to-day difficulties. In the prior art, methods for dealing with ice vary, though most techniques involve some form of scraping, melting or breaking. For example, the aircraft industry utilizes a de-icing solution such as ethyl glycol to douse aircraft wings so as to melt the ice thereon. This process is both costly and environmentally hazardous; however, the risk to passenger safety warrants its use. Other aircraft utilize a rubber tube aligned along the front of the aircraft wing, whereby the tube is periodically inflated to break any ice disposed thereon. Still other aircraft redirect jet engine heat onto the wing so as to melt the ice.
These prior art methods have limitations and difficulties. First, prop-propelled aircraft do not have jet engines. Secondly, rubber tubing on the front of aircraft wings is not aerodynamically efficient. Third, de-icing costs are extremely high, at $2500-$3500 per application; and it can be applied up to about ten times per day on some aircraft.
With respect to many types of objects, resistive DC heating of ice and snow is common. But, heating of some objects is technically impractical. Also, large energy expenditures and complex heating apparati often make heating too expensive.
The invention provides systems and methods for removing or preventing the formation of ice on power lines, airplane wings and other objects.
A system in accordance with the invention for preventing ice and snow on a surface of an object contains an electrical conductor integral with the surface. The conductor is configured to generate an alternating electromagnetic field in response to an AC current. A system also includes a coating integral with both the surface and with the electrical conductor. The coating is configured to generate heat in response to the alternating electromagnetic field. The coating contains a material selected from the group of materials consisting of ferroelectric, lossy dielectric, semiconductor and ferromagnetic materials. A conductor is xe2x80x9cintegralxe2x80x9d with a surface if the surface is within an alternating electromagnetic field generated by an AC current flowing in the conductor. A coating is xe2x80x9cintegralxe2x80x9d with both a conductor and a surface if both the coating is within an alternating electromagnetic field generated by an AC current flowing in the conductor and if the heat generated by the coating prevents ice on the surface. As a practical matter, both conductor and coating are commonly structurally included in the object being protected from ice and snow, for example a power line or an airplane wing. When the heat-generating coating is included in the surface, or is in direct physical contact with the surface, heat transfer between coating and surface is enhanced. Typically, the surface of a conductor itself is being protected; for example, the surface of a conductive airplane wing may be protected by disposing a coating in accordance with the invention on the wing surface and flowing AC current through the wing. The surface of a power line is typically an insulator casing enclosing the main conductors. Conductors may be formed on the surface of the object being protected by various techniques, including photolithography.
In many embodiments in accordance with the invention, for example, in power lines, AC current is already present to generate the alternating electromagnetic field, causing heat in the coating. In other embodiments, a dedicated AC power source may be used to provide AC current; for example, in systems to de-ice airplane wings.
In a typical embodiment, the surface comprises the coating; for example, a coating may adhere permanently to the surface of a power line. In other instances, a coating may be embedded in the object being protected, below the surface exposed to icing; for example, a coating in accordance with the invention may be formed as a layer enclosed within in an airplane wing. Or a coating may be completely separate from the object being protected, being disposed within an integral distance, either permanently or temporarily, to heat the surface of the object.
The coating may be a ferromagnetic material configured to generate heat in response to an alternating magnetic field. Other types of coating may be configured to generate heat in response to a capacitive AC current. In such embodiments, the AC current in the conductor creates an alternating electric field (xe2x80x9cAEFxe2x80x9d), that generates a capacitive AC current in the coating. The capacitive AC current causes heating in the coating. In such embodiments, earth may function as a sink for the capacitive AC current, or another power line may function as a sink, or a special sink may be provided. The coating may comprise semiconductor material configured to generate heat in response to a capacitive AC current. An example of such a semiconductor material is ZnO. The coating may comprise ferroelectric material configured to generate heat in response to a capacitive AC current. Typically, the ferroelectric material has a dielectric constant that changes as a function of temperature, the coating having a low dielectric constant at a temperature above freezing, and a high dielectric constant below freezing. For example, the ferroelectric material may have a Curie temperature, Tc, in the range of from 250xc2x0 to 277xc2x0 K. The coating may comprise lossy dielectric material configured to generate heat in response to a capacitive AC current. The lossy dielectric material may be chosen to have a dielectric loss maximum at an AC frequency in a range of from 40 to 500 Hz when relatively low-frequency AC current is used to prevent icing. On the other hand, the lossy dielectric material may have a dielectric loss maximum at an AC frequency in a range of from 0.5 to 300 kHz when relatively high-frequency AC current is used to prevent ice. For example, if the coating has a dielectric loss maximum at 6 kHz, then the de-icing function can be turned xe2x80x9conxe2x80x9d by switching the AC current from low frequency 60 Hz to 6 KHz. The coating thickness is typically selected to correspond to an amount of heat desired to be generated by the coating. In a particularly simple embodiment, the lossy dielectric material coating is ice itself. In embodiments applied to power lines, the power source typically provides AC current in a range of from 100 to 1000 kV.
An embodiment in accordance with the invention may include a conductive shell, the coating disposed between the electrical conductor and the conductive shell. An example is an aluminum conductive shell surrounding the coating of a power line, thereby forming the outer surface of the power line. By electrically shorting the conductor and the conductived shell when no de-icing is required, the capacitive AC current in the coating is eliminated, no heat is generated by the coating, and no energy is wasted. As with the conductor, the conductive shell may be formed by photolithography. An embodiment typically includes a switch for controlling the electrical connection that shorts conductor and conductive shell. An insulated-gate-bipolar-transistor (xe2x80x9cIGBTxe2x80x9d) power semiconductor switch is well suited. An embodiment typically comprises a control box deriving its power from the alternating electric field. The control box can be remotely controlled; for example, by a radio signal or by a carrier signal. The control box can also be controlled locally and autonomously based on input by a local sensor. For example, the local sensor may include a temperature sensor or an impedance sensor for detecting ice. A typical impedance sensor comprises a 100 kHz impedance sensor. In some embodiments, a control box may comprise a control box case capable of serving as an antenna for gathering energy from the alternating electric field to power the control box. An embodiment may include a number of control boxes, monitoring different sections of the system. For example, a plurality of control boxes may be spaced apart every 5 km or every 50 km along a power line.
An embodiment may include a transformer to transform AC current having a low-voltage to a higher voltage sufficient to generate heat in a coating. Such transformers, for example, may be located at appropriate distance intervals along power lines.
Embodiments in which the coating is ice preferably include a means for frequency-tuning the high-frequency AC current to match the standing-wave effects of ice-dielectric heating and the skin-effect heating resulting from high-frequency current flow in a conductor. An embodiment may also include a means for varying the high-frequency AC current to change the heating pattern produced by standing wave effects of ice-dielectric heating and skin-effect heating, thereby providing sufficient heat at all locations at various times to prevent icing.
In summary, AC current flows through an electrical conductor, creating an electromagnetic field. A coating absorbs the electromagnetic energy, generating heat. The heat from the coating heats the surface of the object being protected above the melting point of ice. The coating material may be a ferroelectric, a lossy dielectric, a semiconductor, or a ferromagnetic material. In some embodiments, the dielectric or magnetic loss properties of the coating depend on a specific temperature. These properties cause the absorption of electromagnetic energy and the resulting heating of the wires only when the ambient temperature drops below the ice""s melting point. In other embodiments, the absorption of energy depends on the frequency of the AC current. A system in accordance with the invention may also include a conductive shell such that the coating material is between the conductor and the conductive shell. By electrically shorting the conductor and the conductive shell, the heating may be switched xe2x80x9coffxe2x80x9d, conserving energy.
In a particular variation, ice itself is utilized as a lossy dielectric coating at high frequency, such as at 60 kHz. Further, skin-effect heating at high frequency may be combined with dielectric heating to melt ice and snow on power lines.
The invention is next described further in connection with preferred embodiments, and it will become apparent that various additions, subtractions, and modifications can be made by those skilled in the art without departing from the scope of the invention.